The present invention relates to the sport of bowling and, more particularly, to apparatus for measuring the imbalance of a bowling ball.
Bowling, as the sport is known in the United States, is conducted according to the rules and standards of the American Bowling Congress. Balls must have a diameter of no more than 27 inches and weigh no more than 16 pounds after the finger holes have been drilled.
The presence of holes dictates that the ball material cannot be of uniform density and still be balanced on all axes passing through its center. Conventionally, a weight, known as the "label weight," is located in the general area where holes are drilled to compensate for the lightening effect of the holes. Some tolerance as to the uniformity of weight distribution is necessary, however, in view of differences in hole sizes. The A.B.C. rules permit maximum amounts of imbalance when measured along various axes of the ball. For example, a maximum difference of three ounces (plus or minus) is permitted between the top half of the ball (the half in which the finger holes are drilled) and the bottom half. This standard allows the ball driller a range of six ounces variation (.+-.3) that he can use, through knowledge of happenstance, to influence the performance of an individual ball. Although permissible imbalance is small when compared to the total weight of the ball, it will be appreciated from a proper understanding of the flight of the ball that this small imbalance is a critical factor in determining ball performance.
As the ball is thrown, it is given velocity in the general direction of the pins and it is given a spin. In most cases the spin axis is not perpendicular to the initial movement of the ball toward the pins. Thus, the ball slides on the alley as it spins but it does not roll, at least not at the beginning of its flight. Eventually, the linear velocity of the ball is reduced, due to frictional forces, and its direction of flight on the alley changes toward alignment with the spin axis. The point at which this change in direction becomes pronounced is called the "roll point." The orientation of the spin axis to the alley when the desired roll point distance is reached is a principal determinant of the ball's ultimate performance.
Since the ball is a heavy spinning mass, it behaves in accordance with those laws of Newtoneon physics known as gyroscope theory. The spin axis remains highly stable at the ball moves down the alley, although there is some tendency for this axis to change due to frictional forces to align itself perpendicular to lateral ball movement. There is also the phenomena of precession to be taken into account.
Precession is caused by the gravity effect on the ball as it rests on its track. If the ball is not perfectly balanced over the track, a torque will result that is opposed by the gyroscopic inertia of the ball. This imbalance torque will cause the spin axis to precess. The inventor terms precession away from the center of the alley "negative" and precession toward the center of the alley "positive."
As the ball rotates on a spin axis not parallel to the alley surface, the center of gravity may move back and forth across the point at which the ball rests on the alley. An oscillatory precession that alternates between positive and negative results. The composite effect may be positive, negative, or neutral.
Skilled ball drillers often attempt to influence the performance of a ball to produce positive or negative precession by drilling the ball in an off-center position with respect to the label, which should, but often does not, accurately indicate the position of the label weight. At times, an additional weight is implanted in the ball at a selected location in an effort to achieve the desired imbalance.
Even after a ball has been drilled and any additional weights have been implanted, it is difficult to empirically determine exactly what has been accomplished. Conventionally, a bowling ball scale is used to statically measure the imbalance. Such a scale determines the added weight that must be suspended from one side of a ball to make it balance on a selected axis and on a selected rest point. This device does not, however, reflect the true imbalance of a ball as it affects the ball's performance. The inventor has determined that the most fundamental shortcomings of this known measuring technique are largely attributable to the inability of such static measurements to properly take into account the fact that the ball contacts the alley on a track that has a smaller diameter than the ball, causing the center of gravity of the ball to shift with respect to its point of contact with the alley as the ball spins. Any static measurement is accurate for only one ball position and can reflect only one of many possible positions of the center of gravity.
Inability to accurately measure the imbalance of a ball in a manner that permits accurate predictions of its performance has resulted in a great deal of mysticism and unscientific thinking in the bowling community. It is often found that a ball simply will not perform in the expected manner. Bowlers are forced to rely on subjective evaluations of a ball and are frustrated by the inability to duplicate a ball that performs favorably.